A description is given to an example of a conventional biosensor in terms of a glucose sensor.
A typical glucose sensor is obtained by forming an electrode system including at least a measuring electrode and a counter electrode on an insulating base plate by screen printing or the like and forming an enzyme reaction layer containing a hydrophilic polymer, oxidoreductase and an electron mediator on the electrode system. For example, glucose oxidase is used as oxidoreductase and a metal complex or an organic compound such as potassium ferricyanide, ferrocene derivatives and quinone derivatives is used as the electron mediator. A buffer is added to the enzyme reaction layer if required.
Upon dropping a sample solution containing a substrate onto the enzyme reaction layer of the biosensor, the enzyme reaction layer dissolves to cause a reaction between the enzyme and the substrate. Reduction of the electron mediator accompanies the reaction. After the enzyme reaction is completed, the reduced electron mediator is electrochemically oxidized to obtain an oxidation current value, from which the substrate concentration in the sample solution is determined.
The biosensor of this kind is theoretically capable of measuring various substances by using an enzyme of which the substrate is a measuring object. For example, if cholesterol oxidase or cholesterol dehydrogenase is used as oxidoreductase, a cholesterol value in serum, which is used as a diagnostic index in various medical institutions, can be measured.
In this case, the enzyme reaction of cholesterol esterase proceeds very slowly. Accordingly, an appropriate surfactant may be added to improve the activity of cholesterol esterase and reduce the time required for the whole reaction. However, the surfactant included in the reaction system affects adversely on hemocytes, which makes impossible to measure whole blood as done in the glucose sensor.
In response to this, a proposal has been made to provide a filter (hemocyte filtering part) in the vicinity of an opening of a sample solution supply pathway so that plasma obtained by filtering hemocytes out of the whole blood is exclusively and rapidly supplied into a sensor (sample solution supply pathway).
However, if the filter is inappropriately built in the sensor, hemocytes captured in the filter are destroyed to dissolve hemoglobin out. Thereby, filtration of the hemocyte components with the filter becomes difficult and small hemoglobin flows into the sample solution supply pathway to cause a measurement error.
This is presumably caused by the fact that a difference in thickness between the filter before absorbing a sample solution and the filter expanded after absorbing the sample solution is not fitted with a gap between pressing parts for holding the filter from the top and the bottom. When the gap between the pressing parts for holding the filter from the top and the bottom is too narrow for the thickness of the expanded filter, the expansion of the filter is prevented. The pore size of the filter thus prevented from expansion cannot be widened sufficiently, thereby the hemocytes as infiltrating thereinto are destroyed.
As opposed to this, if the gap between the upper and lower pressing parts is previously set wide for the supposed thickness of the expanded filter taking into account that the degree of the filter expansion varies depending on a hematocrit value (volume percent of red cell) different in each sample solution, it is feared that the filter may be misaligned during storage of the sensor.
It is also considered that the filter itself is made thinner than a conventional one to prevent the filter from expansion due to the absorption of the sample solution. In this case, however, if the sample solution is sucked only from an end of the filter on a primary side, the amount of the sample solution absorbed within a certain period of time is reduced as described in the specification of Japanese Patent Application No. 2000-399056. Then, the rate at which the plasma flows out of a secondary-side of the filter is reduced and the rate at which the plasma saturates the inside of the sensor, in particular the inside of the sample solution supply pathway, becomes low, which results in long measurement time.
As opposed to this, where a suction area is made wider to increase the amount of the sample solution that can be absorbed within a certain period of time and the sample solution is dropped on an upper part of the filter, the sample solution flows along the surface of the filter at a higher rate than the rate of infiltration into the filter. The sample solution having flown along the filter surface then flows into the sample solution supply pathway from an opening thereof connecting the sample solution supply pathway and the filter, which may cause a measurement error.
In the specification of Japanese Patent Application No. 2001-152868, for example, there is disclosed a technique of providing a first pressing part for holding a primary side portion of the filter from the bottom, second pressing parts for holding a secondary side portion of the filter from the top and the bottom, a third pressing part for holding a center portion of the filter from the top and a void provided between the second and third pressing parts for surrounding the filter. With this technique, the destruction of hemocytes caused by the prevention of the filter expansion is inhibited even if the gap between the pressing parts for holding the filter from the top and the bottom is not fitted with the thickness of the expanded filter. It is also described that the measurement error caused by hemocytes flown into the sample solution supply pathway along the filter surface is avoided by dropping the sample solution directly onto the filter.
In the case of a sensor to which the filter is applied, however, the plasma may spill from an air aperture in some cases depending on the viscosity of the plasma or the plasma amount in whole blood after the filtered plasma flows into and saturates the sample solution supply pathway, though the plasma is expected to stop at the air aperture. The whole blood, which is the sample solution, comprises hemocyte components and a liquid component (plasma), in which the percentage of the hemocyte components (volume percent of red cell) is in the range of about 20 to 60% though it varies among individuals. Further, the viscosity varies depending on the cholesterol concentration in the whole blood. These differences cause a problem in that the plasma travels along the electrode plate to spill from the sample solution supply pathway after reaching the air aperture if the sample solution, even of the same origin, has different flow rate or reduced viscosity and volume percent in red cell.
Hence, the present invention is intended to provide a biosensor that is improved to eliminate the above-described disadvantages and stop the plasma at the air aperture irrespective of the viscosity of the plasma in the sample solution and the plasma amount in the whole blood. Further, the present invention is intended to provide a cholesterol sensor for measuring whole blood with high accuracy and excellent response.